I seldom use the adverbial suffix (-ly) on the adjective first , (either in speaking or writing) because first is recognized an adverb. It me, it's an affectation. But you will see it in texts introducing an introductory paragraph or sentence in a short list of argumentative points: Firstly.....
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coloradayand some prefer first ,even though closely followed by secondly, thirdly, etc.Really? I was taught to use them consistently - I mean, either first, second, third... (more common) or firstly, secondly, thirdly ... (quite formal).